[llvm-dev] llvm.gcroot trouble with non-i8* allocas

[Nikodemus Siivola via llvm-dev]

Solved by using alloca i8*, i32 2 which the system seems happy enough with,
and bitcasting to the actual type for rest of the code after llvm.gcroot.

Not entirely sure if this is a terrible workaround or exactly the way
gcroot is supposed to be used...


On Wed, Nov 1, 2017 at 11:59 AM, Nikodemus Siivola <
nikodemus at random-state.net> wrote:

> I'm allocating { i8*, i32 } on the stack, and would like to add this as a
> GC root, but I'm having trouble figuring this out.
>
> This works as expected:
>
> declare void @llvm.gcroot(i8** %ptrloc, i8* %metadata)
>
> define i8* @bar(i8* %x) gc "shadow-stack" {
> entry:
>   %objptr = alloca i8*
>   call void @llvm.gcroot(i8** %objptr, i8* null)
>   store i8* %x, i8** %objptr
>   %v = load i8*, i8** %objptr
>   ret i8* %v
> }
>
> However, when I have this:
>
> define { i8*, i32 } @foo({ i8*, i32 } %x) gc "shadow-stack" {
> entry:
>   %objptr = alloca { i8*, i32 }
>   %rootptr = bitcast { i8*, i32 }* %objptr to i8**
>   call void @llvm.gcroot(i8** %rootptr, i8* null)
>   store { i8*, i32 } %x, { i8*, i32 }* %objptr
>   %v = load { i8*, i32 }, { i8*, i32 }* %objptr
>   ret { i8*, i32 } %v
> }
>
> llc says: llvm.gcroot parameter #1 must either be a pointer alloca, or
> argument #2 must be a non-null constant.
>   call void @llvm.gcroot(i8** %rootptr, i8* null)
>
> ...and if I do as it asks and add a metadata then I get a crash:
>
> @info = constant i8 1
>
> define { i8*, i32 } @quux({ i8*, i32 } %x) gc "shadow-stack" {
> entry:
>   %objptr = alloca { i8*, i32 }
>   %rootptr = bitcast { i8*, i32 }* %objptr to i8**
>   call void @llvm.gcroot(i8** %rootptr, i8* @info)
>   store { i8*, i32 } %x, { i8*, i32 }* %objptr
>   %v = load { i8*, i32 }, { i8*, i32 }* %objptr
>   ret { i8*, i32 } %v
> }
>
> leads llc into this:
>
> Assertion failed: isa<X>(Val) && "cast<Ty>() argument of incompatible
> type!", file C:\Users\nikod\src\llvm\include\llvm/Support/Casting.h, line
> 236
> Wrote crash dump file "C:\Users\nikod\AppData\Local\
> Temp\llc.exe-7b1546.dmp"
> #0 0x01a26809 HandleAbort c:\users\nikod\src\llvm\lib\
> support\windows\signals.inc:405:0
> #1 0x75804188 (C:\WINDOWS\System32\ucrtbase.dll+0xa4188)
> #2 0x75805422 (C:\WINDOWS\System32\ucrtbase.dll+0xa5422)
> #3 0x75806dc5 (C:\WINDOWS\System32\ucrtbase.dll+0xa6dc5)
> #4 0x75806ce8 (C:\WINDOWS\System32\ucrtbase.dll+0xa6ce8)
> #5 0x7580618b (C:\WINDOWS\System32\ucrtbase.dll+0xa618b)
> #6 0x75806e26 (C:\WINDOWS\System32\ucrtbase.dll+0xa6e26)
> #7 0x013786a6 InsertRootInitializers c:\users\nikod\src\llvm\lib\
> codegen\gcrootlowering.cpp:172:0
> #8 0x01378d97 `anonymous namespace'::LowerIntrinsics::PerformDefaultLowering
> c:\users\nikod\src\llvm\lib\codegen\gcrootlowering.cpp:249:0
> #9 0x01379986 `anonymous namespace'::LowerIntrinsics::runOnFunction
> c:\users\nikod\src\llvm\lib\codegen\gcrootlowering.cpp:195:0
> #10 0x016a343c llvm::FPPassManager::runOnFunction(class llvm::Function &)
> c:\users\nikod\src\llvm\lib\ir\legacypassmanager.cpp:1513:0
> #11 0x016a35e4 llvm::FPPassManager::runOnModule(class llvm::Module &)
> c:\users\nikod\src\llvm\lib\ir\legacypassmanager.cpp:1533:0
> #12 0x016a382c `anonymous namespace'::MPPassManager::runOnModule
> c:\users\nikod\src\llvm\lib\ir\legacypassmanager.cpp:1590:0
> #13 0x016a2e0e llvm::legacy::PassManagerImpl::run(class llvm::Module &)
> c:\users\nikod\src\llvm\lib\ir\legacypassmanager.cpp:1693:0
> #14 0x01000a62 compileModule c:\users\nikod\src\llvm\tools\
> llc\llc.cpp:534:0
> #15 0x010059ca main c:\users\nikod\src\llvm\tools\llc\llc.cpp:289:0
> #16 0x01be99e9 _scrt_common_main_seh f:\dd\vctools\crt\vcstartup\
> src\startup\exe_common.inl:253:0
> #17 0x77178744 (C:\WINDOWS\System32\KERNEL32.DLL+0x18744)
> #18 0x7737582d (C:\WINDOWS\SYSTEM32\ntdll.dll+0x6582d)
> #19 0x773757fd (C:\WINDOWS\SYSTEM32\ntdll.dll+0x657fd)
>
> (This is the same assertion failure that my full code produces, and which
> lead me to trying out things with minimal case and llc.)
>
> What I can see in the debugger isn't immediately illuminating. Any clues?
> Am I missing the obvious here?
>
> This is:
>
> LLVM (http://llvm.org/):
>   LLVM version 4.0.1
>   DEBUG build with assertions.
>   Default target: i686-pc-windows-msvc
>   Host CPU: skylake
>
> Cheers,
>
>  -- nikodemus
>
>
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